Late Quaternary Tephras in the Southern Part of the Tokachi Plain, Hokkaido

Abstract

Kutcharo-Haboro tephra (Kc-Hb), Sinsei tephra (Sns: in this paper), and Azuma-4 tephra (Aafa4) are intercalated in the lower part of the fluvial-peaty sediments which were deposited succeeding the Last Interglacial (Oxygen Isotope Stage 5e) in the southern part of the Tokachi Plain. These three tephras are identified on the basis of petrographic properties such as mineral compositions and refractive indices of volcanic glass, orthopyroxene, and hornblende. The sea level was already at least 25m lower than that of the Last Interglacial maximum transgression when Aafa4, Sns, and Kc-Hb erupted. This stage probably coincided with Oxygen Isotope Stage 5d. The petrographic properties suggest that Sns was derived as a product of the Kutcharo caldera formation, prior to the eruption of Kc-Hb.
Toyoni tephra (Tyn, in this paper) is intercalated as a drift pumice in the marine terrace sediment of Oxygen Isotope Stage 7. Bansei tephra (Bns, in this paper) is intercalated in the lower part of the marine sediment of Oxygen Isotope Stage 9. Although the distribution and the source volcanoes of these two tephras are still unknown, they should be key tephras for examining some Quaternary geologic issues during Oxygen Isotope Stage 7 and 9.